<!DOCTYPE html>
<html>

<head>
    <title>demo</title>
</head>

<body>
</body>
<script type="text/javascript">
var canvas = document.createElement('canvas');
var cxt = canvas.getContext('2d');
var W = canvas.width = 500;
var H = canvas.height = 200;
var str = 'Grewer,点击此处';
cxt.textBaseline = 'hanging';
cxt.font = '60px  宋体'
var sw = cxt.measureText(str).width;
if (sw > W) {
    sw = W;
}
cxt.fillText(str, (W - sw) / 2, (H - 60) / 2, W);

canvas.style.border = '1px solid #ddd'

document.body.appendChild(canvas);

var imageData = cxt.getImageData(0, 0, W, H);

var getV = function(i,j) {
    var v = Math.random() * 1;
    v = v < 0.1 ? 0.1 : v;
    var slope = (i-W/2) / (j-H/2);
    slope =  slope < 0 ? -slope :slope;
    var x = v*slope;
    var y = v;
    if(i<W/2&&j<H/2){
      x = -x;
      y = -y;
    };
    if(i<W/2&&j>H/2){
      x = -x;
    };
    if(i>W/2&&j<H/2){
      y = -y;
    };
    return{x:x,y:y}
}

function getDots(imageData) {
    //从imageData对象中取得粒子，并存储到dots数组中
    var dots = [];
    //dots的索引
    var index = 0;
    for (var i = 0; i < W; i++) {
        for (var j = 0; j < H; j++) {
            //data值中的红色值
            var k = 4 * (i + j * W);
            //data值中的透明度
            if (imageData.data[k + 3] > 0) {
              var v = getV(i,j);
                dots[index++] = {
                    'index': index,
                    'x': i,
                    'y': j,
                    'red': k,
                    'vX': v.x,
                    'vY': v.y,
                }

            }
        }
    }

    var newDots = [];
    var len = dots.length;
    for (var i = 0; i < len; i++) {
        newDots.push(dots.splice(Math.floor(Math.random() * dots.length), 1)[0]);
    }//打乱顺序
    
    return newDots;

}

var dataArr = getDots(imageData)


var random = function() {
    cxt.clearRect(0, 0, W, H);

    for (var i = 0; i < dataArr.length; i++) {
        var temp = dataArr[i];
        temp.x += temp.vX;
        temp.y += temp.vY
        cxt.fillRect(temp.x, temp.y, 1, 1);
    }

    window.requestAnimationFrame(random);
}
document.onclick = function(e) {
    e = e || event;
    var x = e.clientX - canvas.getBoundingClientRect().left;
    var y = e.clientY - canvas.getBoundingClientRect().top;

    if ((0 <= x) && (x <= 500) && (0 <= y) && (y <= 200)) {
        random();
    }
}
</script>

</html>